The use of a sequencing batch reactor (SBR) for wastewater characterization requires the interpretation of the total respirometric response obtained in batch culture with primary settled water. In a first step and with the aim of understanding this total response, the water was divided by filtration into three fractions, the respirometric responses of which were analysed separately. This paper presents the results of the study of the biodegradability and of the specific respirometric responses of the soluble and colloidal fractions of an urban wastewater during batch cultures with a high concentration in pre-acclimatized activated sludges (low So/Xo ratio). For the water studied, the soluble fraction could be divided in two fractions of equal size : a readily biodegradable fraction and an inert fraction. Ultrafiltration of the soluble fraction during the cultures allowed stating the specific evolution of the soluble organic compounds according to their Molecular Weight. For the colloidal fraction, the observed behaviour was totaly different. Indeed, the colloidal organic matter disappeared very quickly from the liquid phase according to a physical phenomenon but without any important biological oxidation during the first 5 hours of a batch culture. Latest work concerns the study of the particulate organic matter and the interpretation of the total respirometric response knowing the specific response of the three fractions.